In one manner of drilling a well, “drilling mud” is pumped into a rotated drill string to which a drill bit is attached. The mud typically exits through openings in the drill bit to lubricate the bit and to carry cuttings up an annulus between the drill string and the wellbore for disposal at the surface. One type of drilling mud is an emulsion of substances which defines a non-aqueous external phase and an aqueous internal phase. In this drilling mud, a non-aqueous “oleaginous” external phase (e.g., oil or synthetic polymers) is used to inhibit swelling of water-sensitive drill cuttings (e.g., shale). It is known in the art that typical oil-based drilling fluids contain some amount of an internal aqueous phase. Typically, an aqueous (water-based) internal phase comprised of salts such as calcium chloride is used to prepare the emulsion structure, imparting viscous properties to the drilling fluid. Additionally, a composition of these oil or synthetic-based drilling fluids typically includes chemical emulsifying agents, which act to form the oleaginous external phase emulsions, also known as “invert” emulsions. These agents also promote oil-wet surfaces. This oil-wetted state promotes lubrication of the drilling bit and further stabilization of formation materials. Such a drilling fluid can be made as known in the art.
After drilling is completed, the drill string or some other string of tubular members (including, for example, liner, casing, or one or more cementing tools) typically is cemented in the wellbore as part of completing the well. One type of cementing operation includes pumping cement down through the string and out into the annulus to displace the drilling mud from the annulus to the surface (however, flow in the opposite direction can occur in some operations, such as in reverse circulating or reverse cementing). A successful cementing operation also includes bonding the cement with the outer surface of the string and the inner surface of the wellbore defining the annulus.
Bonding of the cement to the tubular string or to the wellbore surface can be less than desirable if the string and wellbore surfaces are not conducive to cement bonding. Of particular relevance to the present invention, coatings from the non-aqueous portion of the drilling fluid can interfere with the bonding because the aqueous cement does not bond readily with the non-aqueous substances of the drilling mud. If improper bonding occurs at either of these surfaces, a thin region called a “micro-annulus” can occur. Formation of a micro-annulus can lead to undesirable fluid migration outside the wellbore casing, thus, losing zonal isolation of the wellbore. Further, casing lifetime may be compromised if migrating fluids are corrosive.
To promote proper bonding, an important aspect of oil or gas well cementing is the proper displacement of the drilling fluid from the annular space between the formation and casing or between inner casing and larger outer casing in such a fashion that the formation or casing surfaces may form hydraulic bonds with the cementing slurry. This bond forms best when these surfaces are water-wet. Thus, the displacing fluid must act as an inverter fluid but must also leave the formation or casing surfaces in a water-wet state.
A displacing fluid can be pumped ahead of the cement to create water-wet surfaces. Such fluids include spacers, preflushes, scavenger fluids, or sacrificial slurries, for example. The types of such fluids relevant to the present invention are those which are intended to cause the coatings of non-aqueous (e.g., “oleaginous”) external/aqueous internal drilling muds to invert so that the aqueous phase inverts with the oleaginous phase whereby the aqueous substance is external. These include the primary fluid itself, such as a cement slurry having suitable surfactants. Fluids which cause this inversion are referred to in this description and in the claims as “inverter fluids.” Typically, such inverter fluids are used as a displacement or displacing fluid.
Such inverter fluids are also used to displace oleaginous external/aqueous internal fluids from cased hole or open hole wellbores in operations other than cementing. One example is to replace these fluids with a completion fluid such as a solution of calcium chloride or bromide. This operation is conducted to clean the wellbore for further operation, such as perforation of the casing or, in the case of open hole, production of the well. In this case, the inverter fluid serves to displace the previous fluid and leave the formation surfaces in a water-wet state.